Wafer bonding method for use in making a mems gyroscope

ABSTRACT

A MEMS gyroscope is disclosed herein, wherein the MEMS gyroscope comprised a magnetic sensing mechanism on a magnetic sensor wafer and a magnetic source on a MEMS wafer that further comprises a proof-mass. The magnetic sensor wafer and MEMS wafer are bonded through a bonding mechanism that comprises a hearting mechanism.

CROSS-REFERENCE

This US utility patent application claims priority from co-pending USutility patent application “A HYBRID MEMS DEVICE,” Ser. No. 13/559,625filed Jul. 27, 2012, which claims priority from US provisional patentapplication “A HYBRID MEMS DEVICE,” filed May 31, 2012, Ser. No.61/653,408 to Biao Zhang and Tao Ju. This US utility patent applicationalso claims priority from co-pending US utility patent application “AMEMS DEVICE,” Ser. No. 13/854,972 tiled Apr. 2, 2013 to the sameinventor of this US utility patent application, the subject matter ofeach of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The technical field of the examples to be disclosed in the followingsections is related generally to the art of operation ofmicrostructures, and, more particularly, to operation of MEMS devicescomprising MEMS magnetic sensing structures.

BACKGROUND OF THE DISCLOSURE

Microstructures, such as microelectromechanical (hereafter MEMS) devices(e.g. accelerometers, DC relay and RF switches, optical cross connectsand optical switches, microlenses, reflectors and beam splitters,filters, oscillators and antenna system components, variable capacitorsand inductors, switched banks of filters, resonant comb-drives andresonant beams, and micromirror arrays for direct view and projectiondisplays) have many applications in basic signal transduction. Forexample, a MEMS gyroscope measures angular rate.

A gyroscope (hereafter “gyro” or “gyroscope”) is based on the Corioliseffect as diagrammatically illustrated in FIG. 1. Proof-mass 100 ismoving with velocity V_(d). Under external angular velocity Ω, theCoriolis effect causes movement of the poof-mass (100) with velocityV_(s). With fixed V_(d), the external angular velocity can be measuredfrom V_(d). A typical example based on the theory shown in FIG. 1 iscapacitive MEMS gyroscope, as diagrammatically illustrated in FIG. 2.

The MEMS gyro is a typical capacitive MEMS gyro, which has been widelystudied. Regardless of various structural variations, the capacitiveMEMS gyro in FIG. 2 includes the very basic theory based on which allother variations are built. In this typical structure, capacitive MEMSgyro 102 is comprised of proof-mass 100, driving mode 104, and sensingmode 102. The driving mode (104) causes the proof-mass (100) to move ina predefined direction, and such movement is often in a form ofresonance vibration. Under external angular rotation, the proof-mass(100) also moves along the V_(s) direction with velocity V_(s). Suchmovement of V_(s) is detected by the capacitor structure of the sensingmode (102). Both of the driving and sensing modes use capacitivestructures, whereas the capacitive structure of the driving mode changesthe overlaps of the capacitors, and the capacitive structure of thesensing mode changes the gaps of the capacitors.

Current capacitive MEMS gyros, however, are hard to achievesubmicro-g/rtHz because the capacitance between sensing electrodesdecreases with the miniaturization of the movable structure of thesensing element and the impact of the stray and parasitic capacitanceincrease at the same time, even with large and high aspect ratioproof-masses.

Therefore, what is desired is a MEMS device capable of sensing angularvelocities and methods of operating the same.

SUMMARY OF THE DISCLOSURE

In view of the foregoing, a MEMS gyroscope is disclosed herein, whereinthe gyroscope comprises: a first substrate having a movable portion thatis movable in response to an external angular velocity, said movableportion comprising a magnetic source for generating magnetic field; asecond substrate having a magnetic sensor for detecting the magneticfield from said magnetic source; and a bonding structure for bonding thefirst and second wafer with a predetermined distance, said bondingstructure comprising: a heating mechanism for generating heating; and abonding material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 diagrammatically illustrates the Coriolis effect in a MEMSstructure;

FIG. 2 is a top view of a typical existing capacitive MEMS gyroscopehaving a driving mode and a sensing mode, wherein both of the drivingand sensing mode utilize capacitance structures;

FIG. 3 illustrates an exemplary MEMS gyroscope having a magnetic sensingmechanism;

FIG. 4 illustrates a top view of a portion of an exemplaryimplementation of the MEMS gyroscope illustrated in FIG. 3, wherein theMEMS gyroscope illustrated in FIG. 4 having a capacitive driving modeand a magnetic sensing mechanism;

FIG. 5 illustrates a perspective view of a portion of another exemplaryimplementation of the MEMS gyroscope illustrated in FIG. 3, wherein theMEMS gyroscope illustrated in FIG. 5 having a magnetic driving mechanismfor the driving mode and a magnetic sensing mechanism for the sensingmode

FIG. 6 illustrates an exemplary magnetic driving mechanism of the MEMSgyroscope in FIG. 5;

FIG. 7 illustrates an exemplary magnetic source of the MEMS gyroscopeillustrated in FIG. 3;

FIG. 8 illustrates an exemplary magnetic sensing mechanism that can beused in the MEMS gyroscope illustrated in FIG. 3;

FIG. 9 shows an exemplary thin-film stack that can be configured into aCIP or CPP structure for use in the magnetic sensing mechanismillustrated in FIG. 8;

FIG. 10 illustrates an exemplary MEMS gyroscope that comprises multiplemagnetic sensing structures;

FIG. 11 illustrates an exemplary wafer bonding scheme for use in makingthe MEMS gyroscope shown in FIG. 3; and

FIG. 12 illustrates a perspective view of the water bonding scheme inFIG. 11.

DETAILED DESCRIPTION OF SELECTED EXAMPLES

Disclosed herein is a MEMS gyroscope for sensing an angular velocity,wherein the MEMS gyroscope utilizes a magnetic sensing mechanism. Itwill be appreciated by those skilled in the art that the followingdiscussion is for demonstration purposes, and should not be interpretedas a limitation. Many other variations within the scope of the followingdisclosure are also applicable. For example, the MEMS gyroscope and themethod disclosed in the following are applicable for use inaccelerometers.

Referring to FIG. 3, an exemplary MEMS gyroscope is illustrated herein.In this example, MEMS gyroscope 106 comprises magnetic sensing mechanism114 for sensing the target angular velocity through the measurement ofproof-mass 112. Specifically, MEMS gyroscope 106 comprisesmass-substrate 108 and sensor substrate 110. Mass-substrate 108comprises proof-mass 112 that is capable of responding to an angularvelocity. The two substrates (108 and 110) are spaced apart, forexample, by a pillar (not shown herein for simplicity) such that atleast the proof-mass (112) is movable in response to an angular velocityunder the Coriolis effect. The movement of the proof-mass (112) and thusthe target angular velocity can be measured by magnetic sensingmechanism 114.

The magnetic sensing mechanism (114) in this example comprises amagnetic source 116 and magnetic sensor 118. The magnetic source (116)generates a magnetic field, and the magnetic sensor (118) detects themagnetic field and/or the magnetic field variations that is generated bythe magnetic source (116). In the example illustrated herein in FIG. 3,the magnetic source is placed on/in the proof-mass (112) and moves withthe proof-mass (112). The magnetic sensor (118) is placed on/in thesensor substrate (120) and non-movable relative to the moving proof-mass(112) and the magnetic source (116). With this configuration, themovement of the proof-mass 12) can be measured from the measurement ofthe magnetic field from the magnetic source (116).

Other than placing the magnetic source on/in the movable proof-mass(1112), the magnetic source (116) can be placed on/in the sensorsubstrate (120); and the magnetic sensor (118) can be placed/in theproof-mass (112).

It is also noted that the MEMS gyroscope illustrated in FIG. 3 can alsobe used as an accelerometer.

The MEMS gyroscope as discussed above with reference to FIG. 3 can beimplemented in many ways, one of which is illustrated in FIG. 4.Referring to FIG. 4, the proof-mass (120) is driven by capacitive, suchas capacitive comb. The sensing mode, however, is performed using themagnetic sensing mechanism illustrated in FIG. 3. For this reason,capacitive combs can be absent from the proof-mass (120).

Alternatively, the proof-mass can be driven by magnetic force, anexample of which is illustrated in FIG. 5. Referring to FIG. 5, the masssubstrate (108) comprises a movable proof-mass (126) that is supportedby flexible structures such as flexures 128, 129, and 130. The layout ofthe flexures enables the proof-mass to move in a plane substantiallyparallel to the major planes of mass substrate 108. In particular, theflexures enables the proof-mass to move along the length and the widthdirections, wherein the length direction can be the driving modedirection and the width direction can be the sensing mode direction ofthe MEMS gyro device. The proof-mass (126) is connected to frame 132through flexures (128, 129, and 130). The frame (132) is anchored bynon-movable structures such as pillar 134. The mass-substrate (108) andsensing substrate 110 are spaced apart by the pillar (134). Theproof-mass 12) in this example is driving by a magnetic drivingmechanism (136). Specifically, the proof-mass (126) can move (e.g.vibrate) in the driving mode under magnetic force applied by magneticdriving mechanism 136, which is better illustrated in FIG. 6.

Referring to FIG. 6, the magnetic driving mechanism 136 comprise amagnet core 138 surrounded by coil 140. By applying an alternatingcurrent through coil 140, an alternating magnetic field can be generatedfrom the coil 140. The alternating magnetic field applies magnetic forceto the magnet core 140 so as to move the magnet core. The magnet corethus moves the proof-mass.

The magnetic source (114) of the MEMS gyroscope (106) illustrated inFIG. 3 can be implemented in many ways, one of which is illustrated inFIG. 7. Referring to FIG. 7, conductive wire 142 is displaced on/inproof-mass 112. In one example, conductive wire 142 can be placed on thelower surface of the proof-mass (112), wherein the lower surface isfacing the magnetic sensors (118 in FIG. 3) on the sensor substrate(110, in FIG. 3). Alternatively, the conductive wire (142) can be placedon the top surface of the proof-mass (112), i.e. on the opposite side ofthe proof-mass (112) in view of the magnetic sensor (118). In anotherexample, the conductive wire (142) can be placed inside the proof-mass,e.g. laminated or embedded inside the proof-mass (112), which will notbe detailed herein as those examples are obvious to those skilled in theart of the related technical field.

The conductive wire (142) can be implemented in many suitable ways, oneof which is illustrated in FIG. 7. In this example, the conductive wire(142) comprises a center conductive segment 146 and tapered contacts 144and 148 that extend the central conductive segment to terminals, throughthe terminals of which current can be driven through the centralsegment. The conductive wire (142) may have other configurations. Forexample, the contact tapered contacts (144 and 148) and the centralsegment (146) maybe U-shaped such that the tapered contacts may besubstantially parallel but are substantially perpendicular to thecentral segment, which is not shown for its obviousness.

The magnetic sensor (118) illustrated in FIG. 3 can be implemented tocomprise a reference sensor (150) and a signal sensor (152) asillustrated in FIG. 8. Referring to FIG. 8, magnetic senor 118 on/insensor substrate 120 comprises reference sensor 150 and signal sensor152. The reference sensor (150) can be designated for dynamicallymeasuring the magnetic signal background in which the target magneticsignal (e.g. the magnetic field from the conductive wire 146 asillustrated in FIG. 7) co-exists. The signal sensor (152) can bedesignated for dynamically measuring the target magnetic signal (e.g.the magnetic field from the conductive wire 146 as illustrated in FIG.7). In other examples, the signal sensor (152) can be designated fordynamically measuring the magnetic signal background in which the targetmagnetic signal (e.g. the magnetic field from the conductive wire 146 asillustrated in FIG. 7) co-exists, while the signal sensor (150) can bedesignated for dynamically measuring the target magnetic signal (e.g.the magnetic field from the conductive wire 146 as illustrated in FIG.7).

The reference sensor (150) and the signal sensor (152) preferablycomprise magneto-resistors, such as AMRs, giant-magneto-resistors (suchas spin-valves, hereafter SV), or tunneling-magneto-resistors (TMR). Fordemonstration purpose, FIG. 9 illustrates a magneto-resistor structure,which can be configured into CIP (current-in-plane, such as aspin-valve) or a CPP (current-perpendicular-to-plane, such as IMP,structure). As illustrated in FIG. 9, the magneto-resistor stackcomprises top pin-layer 154, free-layer 156, spacer 158, reference layer160, bottom pin layer 162, and substrate 120. Top pin layer 154 isprovided for magnetically pinning free layer 156. The top pin layer canbe comprised of IrMn, PtMn or other suitable magnetic materials. Thefree layer (156) can be comprised of a ferromagnetic material, such asNiFe, CoFe, CoFeB, or other suitable materials or the combinationsthereof. The spacer (158) can be comprised of a non-magnetic conductivematerial, such as Cu, or an oxide material, such as Al₂O₃ or MgO orother suitable materials The reference layer (160) can be comprised of aferromagnetic magnetic material, such as NiFe, CoFe, CoFeB, or othermaterials or the combinations thereof. The bottom pin layer (162) isprovided for magnetic pinning the reference layer (160), which can becomprised of a IrMn, PtMn or other suitable materials or thecombinations thereof. The substrate (120) can be comprised of anysuitable materials, such as glass, silicon, or other materials or thecombinations thereof.

In examples wherein the spacer (158) is comprised of a non-magneticconductive layer, such as Cu, the magneto-resistor (118) stack can beconfigured into a CIP structure (i.e. spin-valve, SV), wherein thecurrent is driven in the plane of the stack layers. When the spacer(158) is comprised of an oxide such as Al₂O₃, MgO or the like, themagneto-resistor stack (118) can be configured into a CPP structure(i.e. TMR), wherein the current is driven perpendicularly to the stacklayers.

In the example as illustrated in FIG. 9, the free layer (156) ismagnetically pinned by the top pin layer (154), and the reference layer(160) is also magnetically pinned by bottom pin layer 162. The top pinlayer (154) and the bottom pin layer (162) preferably having differentblocking temperatures. In this specification, a blocking temperature isreferred to as the temperature, above which the magnetic pin layer ismagnetically decoupled with the associated pinned magnetic layer. Forexample, the top pin layer (154) is magnetically decoupled with the freelayer (156) above the blocking temperature T_(B) of the top pin layer(154) such that the free layer (156) is “freed” from the magneticpinning of top pin layer (154). Equal to or below the blockingtemperature T_(B) of the top pin layer (154), the free layer (156) ismagnetically pinned by the top pin layer (154) such that the magneticorientation of the free layer (156) is substantially not affected by theexternal magnetic field. Similarly, the bottom pin layer (162) ismagnetically decoupled with the reference layer (160) above the blockingtemperature T_(B) of the bottom pin layer (162) such that the referencelayer (160) is “freed” from the magnetic pinning of bottom pin layer(162). Equal to or below the blocking temperature T_(B) of the bottompin layer (162), the reference layer (160) is magnetically pinned by thebottom pin layer (162) such that the magnetic orientation of thereference layer (162) is substantially not affected by the externalmagnetic field.

The top and bottom pin layers (154 and 162, respectively) preferablyhave different blocking temperatures. When the free layer (156) is“freed” from being pinned by the top pin layer (154), the referencelayer (160) preferably remains being pinned by the bottom pin layer(162). Alternatively, when the free layer (156) is still pinned by thetop pin layer (154), the reference layer (160) can be “freed” from beingpinned by the bottom pin layer (162). In the later example, thereference layer (160) can be used as a “sensing layer” for responding tothe external magnetic field such as the target magnetic field, while thefree layer (156) is used as a reference layer to provide a referencemagnetic orientation.

The different blocking temperatures can be accomplished by usingdifferent magnetic materials for the top pin layer (154) and bottom pinlayer (162). In one example, the top pin layer (154) can be comprised ofIrMn, while the bottom pin layer (162) can be comprised of PtMn, viceversa. In another example, both of the top and bottom pin layers (154and 162) may be comprised of the same material, such as IrMn or PtMn,but with different thicknesses such that they have different blockingtemperatures.

It is noted by those skilled in the art that the magneto-resistor stack(118) is configured into sensors for sensing magnetic signals. As such,the magnetic orientations of the free layer (156) and the referencelayer (160) are substantially perpendicular at the initial state. Otherlayers, such as protective layer Ta, seed layers for growing the stacklayers on substrate 120 can be provided, It is further noted that themagnetic stack layers (118) illustrated in FIG. 9 are what is oftenreferred to as “bottom pin” configuration in the field of art. In otherexamples, the stack can be configured into what is often referred as“top pinned” configuration in the field of art, which will not bedetailed herein.

In some applications, multiple magnetic sensing mechanisms can beprovided, an example of which is illustrated in FIG. 10. Referring toFIG. 10, magnetic sensing mechanisms 116 and 164 are provided fordetecting the movements of proof-mass 112. The multiple magnetic sensingmechanisms can be used for detecting the movements of proof-mass 112 indriving mode and sensing mode respectively. Alternatively, the multiplemagnetic sensing mechanisms 116 and 164 can be provided for detectingthe same modes (e.g. the driving mode and/or the sensing mode).

The MEMS gyroscope as discussed above can be fabricated in many ways.During fabrication, special concerns on the fact that magnetic sensorand MEMS proof-mass may having different properties need to beaddressed. In some examples, the magnetic sensor comprises spintronicstructures, such as spin-valve (SV), magnetic-tunnel-junction (MTJ) orother similar structures. These spintronic structures in general has ablocking temperature that is 220 C or less, which means that thesespintronic structures can be processed at a temperature mot higher thanthe blocking temperature. However, the MEMS structures generally areprocessed at a much higher temperature, such as 250 C or higher. INparticular, bonding the MEMS wafer having the proof-mass and themagnetic sensor wafer having the magnetic sensor generally requires abonding temperature of 250 C or higher to secure a reliable bondingstrength. This problem can be solved by using a localized heating, asillustrated in FIG. 11.

Referring to FIG. 11, MEMS wafer 108 has proof-mass 112, on whichmagnetic source 116 is provided. Magnetic sensor wafer 110 has magneticsensor 118. MEMS wafer 108 and magnetic sensor wafer 110 can be bondedby using bonding structure 170 that comprises a pillar (e.g. pillar 172and/or 174), localized heater 176 and bonding material 178. FIG. 12shows a perspective view of the structure in FIG. 11 to betterillustrate the structure.

Referring to FIG. 12, localized heater 176 is disposed on the surface ofpillar 174 wherein pillar 174 in this example forms a hermetic bondingeven though not required in some examples. The localized heater isconductive such that it generates localized heating to raise thetemperature in the vicinity of the heater when current is driventhrough. The localized heater has two terminals 182 fur feeding current.Bonding material 178, which can be any suitable bonding materials, suchas a glass frit, metal alloy, or metal can be disposed on the heater. Inthis way, the bonding material (178) and the pillar (174) laminate thelocalized heater therebetween. In some examples especially when thebonding material is conductive, an insulating layer can be disposedbetween the localized heater (176) and bonding material 178, which isnot shown in the figure.

In a bonding process, current is driven into the localized heaterthrough terminals 182. As current flows through the localized heater,the temperature in the vicinity of the heater (176) is elevated. Whenthis temperature equals to or higher than the melting temperature of thebonding material 178, the pillar (174) can be bonded to the magneticsensor wafer (120) (which is not shown in FIG. 12). In this bondingprocess, pressure can be applied.

By using the localized heater (176), only the area in the vicinity ofthe heater (176) raises its temperature. The magnetic sensor may notexperience temperature raise. As such, the bonding area (the areawherein the bonding material is disposed) can have a temperature higherthan 220 C, such as 350 C or 400 C, while the area wherein the magneticsensor is disposed may still keep a safe temperature that is lower than200 C.

the example shown in FIG. 12, pillar 174 forms a hermetic bonding. Thesame bonding scheme and process are also applicable examples wherein theMEMS wafer and the magnetic sensor wafer are bonded non-hermetically. Inthose examples, multiple pillars can be provided; and each bonding areaat individual pillars may be provided with a localized heater forgenerating localized heating.

The pillar (172, 174) can be separately provided. In another example,the pillar can be directly formed from the MEMS wafer (108) shown inFIG. 11. Alternatively, the pillar can be formed from the magneticsensor wafer, in examples of which, the localized heater and the bondingmaterials can be disposed in the vicinity of the MEMS wafer (108).

It will be appreciated by those of skilled in the art that a new anduseful MEMS gyroscope has been described herein. In view of the manypossible embodiments, however, it should be recognized that theembodiments described herein with respect to the drawing figures aremeant to be illustrative only and should not be taken as limiting thescope of what is claimed. Those of skill in the art will recognize thatthe illustrated embodiments can be modified in arrangement and detail.Therefore, the devices and methods as described herein contemplate allsuch embodiments as may come within the scope of the following claimsand equivalents thereof. In the claims, only elements denoted by thewords “means for” are intended to be interpreted as means plus functionclaims under 35 U.S.C. §112, the sixth paragraph.

We claim:
 1. A MEMS gyroscope, comprising: a first substrate having amovable portion that is movable in response to an external angularvelocity, said movable portion comprising a magnetic source forgenerating magnetic field; a second substrate having a magnetic sensorfor detecting the magnetic field from said magnetic source; and abonding structure for bonding the first and second wafer with apredetermined distance, said bonding structure comprising: a heatingmechanism for generating heating; and a bonding material.
 2. The MEMSgyroscope of claim 1, wherein the bonding structure further comprises aninsulating layer between the heating mechanism and the bonding material.3. The MEMS gyroscope of claim 2, wherein the bonding material comprisesa metal or a metal alloy.
 4. The MEMS gyroscope of claim 2, wherein themagnetic sensor comprises a giant-magnetic-resistor.
 5. The MEMSgyroscope of claim 2, wherein the magnetic sensors comprises aspin-valve structure.
 6. The MEMS gyroscope of claim 2, wherein themagnetic sensors comprises a tunnel-magnetic-resistor.
 7. The MEMSgyroscope of claim 2, wherein the magnetic sensors comprises a magneticpickup coil that is an element of a fluxgate.